What is DCOM ?
DCOM (Distributed COM) is an integral security mechanism in the Windows operating system. DCOM is defined as follows in the Windows Help file:
"Distributed component object model (DCOM) is a network extension of the component object model (COM) technology that enables interprocess communication across the network. DCOM supports communication between objects on a network by handling low-level details of network protocols. This enables the use of distributed programs consisting of multiple processes working together to accomplish a single task."
In other words, DCOM can make distributed applications secure without any security-specific coding or design in either the client or the component (in this case, the IGSS OPC Server).
The IGSS OPC Server uses DCOM
When the 3rd party OPC Client connects to the IGSS OPC Server, DCOM is used as the security layer to grant the client access to the server. The figure below shows how this works.
The DCOM Configuration Tool
The DCOM security settings are defined in the DCOM Configuration Properties utility in Windows. Start the utility by choosing Start ® Run and then typing Dcomcnfg. The examples in this help file shows the dialog boxs from Windows 2000, but the user interface has changed in Windows XP.
There are two sets of DCOM security settings:
The default security settings for all DCOM applications on the computer
The specific security settings for the individual DCOM application on the computer
In our recommended procedure, we use the latter method to set up the IGSS OPC Server specifically. Of course, if you want all DCOM applications to share the same security settings, simply alter the default security settings for all applications.
The Distributed DCOM Configuration Properties dialog box in Windows 2000
Further reading about DCOM
The OPC Foundation has issued a number of white papers related to DCOM security. Click the links below to view the documents in .pdf format: